The use of feedthroughs, or conductive vias, is common in electronic fabrication and packaging where electrical continuity or electrical connection is required between two sides of a non-conductive body, where such body also forms part of a housing or package for electronic circuitry. Such bodies, when produced in a substantially planar configuration and configured for attachment of electronic circuitry, are commonly referred to as “substrates,” but may be utilized in other geometric configurations, such as cylinders, cubes, spheres, or sections thereof, with or without attached circuitry. Feedthroughs are particularly suited for use in electrochemical sensing devices where electrodes on one side of a substrate need to communicate electrically with circuitry that is physically in communication with the other side of the substrate. Such circuitry often includes integrated circuits. In many electrochemical sensing devices, such as those designed for implantation in humans, the feedthrough must also be a moisture-resistant barrier in order to protect the electronic circuitry from moisture damage due to the fact that the electrodes come into contact with body fluids. Acceptably impervious barrier structures are commonly referred to as “hermetic,” and the term is often associated with a transmission rate for helium of 10−8 cc-atm/sec or less, as in, for example, MIL-STD 883D. Feedthroughs also find application in vacuum systems, in the construction of batteries, and in various types of instrumentation.
Ceramic is widely used as a material for mounting and/or housing electronic circuitry. It is also a dielectric and can therefore serve as a mounting surface for various electrical contacts and electronic components which can be electrically connected using conductive traces, solder, conductive pastes, wire bonding, etc. Many types of ceramics are also biocompatible materials, making them suitable for implantation in the human body. In the past, various methods have been developed for providing feedthroughs in ceramic bodies by the addition of discrete, multi-component assemblies that include metal, glass, and ceramic elements. Such feedthrough assemblies are complex, costly, and unsuitable for fabrication into high-density multi-conductor configurations.
Ceramic materials can be readily formed and machined when in the so-called “green” state. The green state is a form of the material comprising a mixture of the ceramic in particulate form and a volatile organic binder that supports the particles. The presence of this organic binder is also an indicator of whether the ceramic material has been fired. An example of such material, and methods for forming it into desired shapes, is described in U.S. Pat. No. 5,487,855 of Moeggenborg, et al. assigned to Nalco Chemical Company.
Many ceramic materials are supplied commercially in the green state. During the firing process, the polymer binder material is driven off and/or decomposed at elevated temperature, allowing the ceramic particles to closer approximate each other and become sintered or fused. The loss of volatile binder material and the sintering of the ceramic particles in the firing process lead to a shrinkage of the part during the firing process. Ceramic materials typically have very high strength after firing, but a much lower strength prior to firing, when in the green state. Depending on the formulation, green ceramic materials may possess varying degrees of ductility.
U.S. Pat. Nos. 5,855,995 and 6,041,496 of Haq et al. assigned to Medtronic, Inc., disclose a ceramic substrate for an implantable medical device such as a heart pacemaker. The substrate includes a stack of ceramic layers. External metallization layers are connected by internal metallization layers and paste-filled vias that are fired to achieve hermeticity. The paste contains 20% or less by weight high temperature ceramic powder and about 80% or more by weight powdered ruthenium, platinum, or other metals. Upon firing at a temperature between 1400° C. and 1800° C., the ceramic powder in the via-fill paste causes the via-fill material to adhere to the walls of the via.
U.S. Pat. No. 5,273,203 of Webster assigned to General Electric Company discloses a hermetic seal for a conductive feedthrough through a ceramic component. The feedthrough comprises a small lead made of a platinum or palladium core with a thin copper plating which is surrounded by a copper collar. A copper-copper oxide eutectic seals the gap between the lead and the walls of the aperture in the ceramic through which the lead extends. The requirement of a potentially corrodible copper component is unacceptable for applications in which the external side of the feedthrough functions as an electrode in an aqueous environment.
U.S. Pat. Nos. 5,821,011 and 6,090,503 of Taylor et al. assigned to Medtronic, Inc. disclose a body implanted device with an electrical feedthrough in the wall of a titanium or titanium alloy container. The patent claims a center pin or terminal that is surrounded by a special glass material which is highly resistant to the corrosive effects of organic electrolytes found in batteries or the corrosive effects of direct contact with body fluids. This construction requires the incorporation of a glass material to promote the seal around the center pin.
U.S. Pat. No. 6,221,513 of Lasater assigned to Pacific Coast Technologies, Inc. discloses methods for hermetically sealing an interface of ceramic materials to an interface surface of titanium-containing alloys using a titanium-nickel alloy filler material. The filler material is additionally in contact with a ceramic component containing zirconia, whereby the titanium-nickel material forms a liquid at less than 1100° C. in the presence of the metallic and ceramic components.
U.S. Pat. No. 5,782,891 of Hassler et al. also assigned to Medtronic, Inc. discloses a packaging arrangement for an implantable medical device including a ceramic enclosure and a multi-layered substrate having multiple feedthroughs. The multi-layered substrate couples to the ceramic enclosure at the edges around an opening.
U.S. Pat. Nos. 5,283,104 and 5,337,475 of Aoude et al. assigned to International Business Machines Corporation disclose compositions for producing conductive vias in multi-layer ceramic substrates having circuits, without cracking and/or loss of hermetic sealing. A via paste mixture is introduced in the via. The mixture contains glass spheres smaller than 10% of the via size and metallic spheres less than ⅓ the size of the glass spheres. The metallic spheres are made of copper, copper-beryllium, copper-iron-cobalt alloys, or other materials. Such metals are not appropriate for use with electrochemical sensors in contact with electrolytic fluids, and the vias formed by this process could not serve directly as electrodes. Appropriate electrode materials such as platinum are not included.
U.S. Pat. No. 6,812,404 of Martinez assigned to Medtronic, Inc. discloses feedthrough assemblies and methods for creating feedthrough assemblies with brazed seals and a conductive material that provides electrochemical corrosion protection of the brazed seals. The assemblies require the braze material to provide the hermetic sealing between the terminals and the surrounding insulators and further require another electrically conductive material to protect the braze material from the corrosive effects of contact with aqueous fluids.
U.S. Pat. Nos. 5,046,242, 5,105,811, and 5,272,283 of Kuzma assigned to the Commonwealth of Australia disclose methods for creating feedthrough assemblies and a cochlear prosthetic package that utilizes these feedthroughs. The feedthrough assemblies rely on hollow tubes that are placed in the ceramic bodies and are welded closed on at least one end to provide a hermetic assembly.